New Concept for HLCT Emitter: Acceptor Molecule in Exciplex System for Highly Efficient and Extremely Low-Efficiency Roll-Off Solution-Processed OLED

The use of hybridized local and charge-transfer (HLCT) emitters as acceptor or donor molecules in exciplex systems is still a novel concept even though HLCT molecules can harvest both singlet and triplet excitons. Herein, two HLCT emitters, 2-^tBuspoCz-2pTrz and 2-^tBuspoCz-Me3pTrz, are prepared as acceptors of exciplexes with thermally activated delayed fluorescence (TADF) properties. The exciplex TADF systems constructed by mixing HLCT and TAPC molecules have a minute energy gap between singlet and triplet excited states and unity emission efficiency. Using the exciplex as an emitter, solution-processed devices achieve maximum external quantum efficiency (EQE_max) of 16.6%. Impressively, solution-processed devices with the fabricated exciplexes as hosts exhibit extremely low turn-on voltages and a promising EQE_max of >20%, which is concomitant with an extremely low-efficiency roll-off of 0.5% at 1,000 cd m^–2. This study explores the potential of HLCT emitters as acceptors of exciplexes and guides the design of efficient exciplex systems.